The Potential Role of Megakaryocytes in Mechanically Mediated Bone Adaptation.

Abstract

Maintenance of bone mass and geometry is heavily dependent upon mechanical stimuli. Current paradigms suggest that osteocytes embedded within the mineralized matrix and osteoblasts on the bone surfaces are the primary responders to physical forces. However, other cells within the marrow cavity are also subject to a mechanically active environment. Megakaryocytes (MKs), cells which produce platelets, may physiologically be exposed to fluid shear forces. Recent studies have highlighted the potent effects MKs have on osteoblast proliferation as well as bone formation in vivo. We hypothesize that MKs are capable of responding to physical forces and that the interactions between these cells and osteoblasts can be influenced by mechanical stimulation. We have demonstrated that two MK cell lines respond to fluid shear stress in culture. Furthermore, we isolated MKs from histologic sections of murine tibiae that were exposed to compressive loads in vivo using laser capture microdissection. C-fos, a transcription factor shown to be upregulated in response to load in various tissue types, was increased in MKs from loaded relative to non-loaded limbs at a level comparable to that of osteocytes from the same limbs. To assess the functional outcomes of this mechanoresponsiveness, we first set out to determine whether animals with elevated numbers of MKs demonstrated altered adaptation to mechanical loading. GATA-1low mice, a transgenic mouse model with arrested MK maturation leading to an elevated number of immature MKs within the marrow, were shown to have a minimally altered response to load compared to wild-type littermates. Mice injected with thrombopoietin, a potent inducer of MK proliferation and differentiation, showed no difference in response to load compared with vehicle-injected mice. Finally, we developed a co-culture system to address whether mechanical stimulation of MKs in culture would impact osteoblast proliferation and differentiation. The presence of MKs in culture, but not conditioned media, has dramatic effects on osteoblast proliferation. Our data suggests a minimal, but non-significant, decrease in proliferation as well as an increase in mineralization capacity of osteoblasts co-cultured with MKs exposed to shear compared to co-cultures with unstimulated MKs. Further studies are necessary to investigate the mechanism driving this phenomenon.